The digital data (IEC61937 format) output from a digital audio terminal of a DVD (digital versatile disk) reproduction apparatus includes, like in the conventional CD (compact disk), compressed audio data of various formats such as AC-3 (trade mark) proposed by Dolby, MPEG and dts (trade mark) proposed by DTS, in addition to the linear PCM (pulse code modulation). An external decoder unit connected to the DVD reproduction apparatus, therefore, is required to determine these data formats and accurately process the data.
These compressed audio data, as viewed from the decoder unit, are simply the conventional 16-bit PCM data, and until the sync signal in the burst preamble existing in the 16-bit data is detected, it cannot be determined whether the particular data are actually PCM data that can be demodulated or the compressed audio data requiring the decode processing such as expansion processing.
This determination is possible to some degree for the DVD reproduction signal by detecting the PCM flag or the channel status on the IEC61937 format. The CD recorded in the dts format recently marketed in the U.S., however, is handled as a normal CD (compact disk) also in the DVD reproduction apparatus, and therefore the PCM data is determined according to the PCM flag with a risk.
In view of this, in the conventional decoder apparatus, the bit pattern of input data is always compared with a conceivable sync pattern, and upon detection of any sync signal indicating compressed audio data, the audio decoding of the PCM data is suspended, and if the data can be decoded, the decoder apparatus is activated, thereby preventing the noise which otherwise might be caused by the decoding error of the compressed audio data.
Japanese Unexamined Patent Publication No. 5-316056 discloses an audio signal processing apparatus which, in order to reduce noises and click sound, detects the number of data input of a value not contained in the conversion rule, and in the case where the detection result reaches a predetermined value or more, controls the output sound into muted status.
Japanese Unexamined Patent Publication No. 8-287613 discloses an output control system of a switchable audio channel, in which in order to suppress noises at the time of switching the audio channel, one (audio channel 1) of a plurality of types of audio data is changed to another type (audio channel 2), while the audio output of audio channel 2 is faded in after fading out the audio output of audio channel 1.